In the manufacture of semiconductor devices, photolithography is often used, especially in view of the circuit patterns of semiconductors being increasingly miniaturized in recent years. Projection optics are used to image a mask or reticle onto a wafer and as circuit patterns have become increasingly smaller, there is an increased demand for higher resolving power in exposure apparatuses that print these patterns. To satisfy this demand, the wavelength of the light source must be made shorter and the NA (numerical aperture) of the optical system (i.e., the projection lens) must be made larger.
One way to describe an optic system is by the location and/or number of waists that are formed throughout the system. For example, a 1½ waist system is characterized by using two negative lens groups in minimum. Each of them comprises a minimal two negative lenses. The first of these groups with negative power is located at the beginning of the lens system. The overall length of this group is smaller then 15% of the overall length of the complete lens system. Furthermore this lens group has a positive lens as the first lens element. The usual used term ‘waist’ characterizes the shape of the light beam running through the system. At a waist the beam presents a local minimum in the diameter of the beam. It is characterized by the upper coma ray that changes its direction from convergent to divergent inside a negative lens group. The minimum beam diameter at a waist is smaller then about 90% of the maximum beam diameter before and after the waist. This means the maximum beam diameters before and after that waist are larger then about 110% of the minimum beam diameter at the waist. In view of this, a half waist is understood as a more shallow change of the beam diameter. At a half waist, the change of direction of the upper coma ray is remarkable smaller than in a regular waist. The minimum beam diameter inside the waist is only smaller then about 95% of the beam diameter after the waist. The maximum beam diameter is increasing only after the waist to values that are larger then about 105% of the minimum beam diameter at the half waist. The foregoing discussion defines the term “1½ waist” as used throughout the present specification.
The prior art does include some examples of dioptric projection systems for DUV lithography that have both a large numerical aperture and a large field (e.g. 26 mm×8 mm or larger) suitable to support step and scan lithography. For example, U.S. Pat. Nos. 6,008,884 and 6,259,508; European patent application EP 1061396A2 and European patent application EP 1139138A1 disclose dioptric projection systems. Each of these documents is incorporated herein by reference in its entirety. The '884 and the '508 patents disclose that no aspheres be used in or after the primary waist, with the exception of aspheres located in close proximity to the wafer. In addition, these patents disclose that no aspheres are present in the positive lens group that follows the principle waist. FIG. 1 is a schematic illustration of one exemplary dioptric projection system 10 that is disclosed in the '508 patent. The '396 patent application places limitations of asphere behavior, specifically excluding aspheric surface pairs whose “local curvature powers change with mutually opposite signs.” The '138 application discloses and describes various shapes that can be used for the asphere in such a system (e.g., no aspheres on concave surfaces). More specifically, the '138 application discloses that the local principal curvatures Ca and Cb are computed as follows:             C      a        =          1      r                  C      b        =                                        ⅆ            2                    ⁢          z                                      ⅆ            2                    ⁢          y                                      [                      1            +                                          (                                                      ⅆ                    z                                                        ⅆ                    y                                                  )                            2                                ]                          2          /          3                    In equations (1) Ca is the local curvature at the vertex of the asphere and Cb is the local curvature in the tangential or meridional plane at an extreme region of the clear aperture, r is the vertex radius and the first and second derivative of z are taken from standard aspheric sag equation. The '138 application lists limitations for the system that are based on a ratio of Cb/Ca for both positive and negative surfaces in the region closest to the mask.
In addition, U.S. Pat. Nos. 6,560,031 and 6,590,715 disclose optical projection lens systems that include one or more waists in each system. The '031 and '715 patents are assigned to the present assignee and offer alternative design constructions; however, none of these systems discloses the present system and provides the associated advantages and benefits disclosed herein.
What has heretofore not been available is a compact 1½ waist projection system having reduced axial chromatic aberration and offering an alternative construction relative to the above mentioned known systems.